Laser-ablatable flexographic printing elements are known in principle. They comprise at least a dimensionally stable support, a photopolymerizable layer and a laser-ablatable layer, also called an LAMS (laser-ablatable mask system). The laser-ablatable layer is generally protected by a peelable protective sheet known as the coversheet. The photopolymerizable layer may be constructed of water-soluble components, but particularly organosolvent-soluble photopolymerizable layers are widely used. Flexographic printing elements of this type are also referred to as digitally imageable flexographic printing elements.
Flexographic printing forms are made from photopolymerizable laser-ablatable flexographic printing elements via a multistepped process using dedicated equipment for each step. So the process is complex and time consuming.
First the coversheet is peeled off and a mask is written into the laser-ablatable layer by using an IR laser. Laser apparatus featuring a rotating drum, flat-bed apparatus or internal-drum lasers may be concerned at this stage.
After the mask has been written into the laser ablatable layer, the flexographic printing element is exposed to UV or UV/VIS radiation through the mask formed.
The photopolymerizable layer undergoes polymerization in the areas no longer concealed by the mask, whereas no polymerization occurs in the concealed areas. This is done using UV exposure units, which may comprise various sources of UV, for example UV tubes or UV LEDs.
After exposure, the remnants of the mask and also the unpolymerized portions of the photopolymerizable layer are removed. This can be done using one or more solvents or else thermally. The washout with solvents is done using specific types of washout equipment, for example brush washers. Thermal development may utilize equipment where an absorbent nonwoven is pressed by a heated roller against the exposed layer.
In the course of washout by means of organic solvents, the unpolymerized portions of the relief-forming layer dissolve in the solvent. The washout media used are typically mixtures comprising high-boiling hydrocarbons (boiling range typically about 150° C. to 200° C.). The polymerized portions of the layer do not dissolve and remain after the washout step to form the printing relief. But they do nonetheless swell in the washout media. When solvents are used to develop the plate, therefore, a drying step in a dryer follows. The drying step is typically carried out in circulating air dryers at a temperature of not more than 60° C.
After the drying step, the surface of the flexographic printing form obtained is usually aftertreated, for example by exposing the surface to UVA and/or UVC radiation. This again requires dedicated exposure equipment.
The rate-determining step in the making of flexographic printing plates is the step of drying the washed flexographic printing plates. Exposure, washout and after-treatment generally require from 10 to 20 minutes each. The length of the drying step, by contrast, varies with the plate type used and particularly the plate thickness in the range from at least 60 minutes up to 3 or 4 hours. The long drying period in the case of thick flexographic plates in particular is needed in order to effect ideally quantitative removal from the flexographic printing plate of even residues of the high-boiling washout media. To determine the drying period, it is customary to measure the layer thickness of the flexographic printing plate before and after washout and then compare it with the layer thickness of the flexographic printing plate after different drying periods. Drying can also be tracked via the decreasing weight of the flexographic printing plate during drying.
Residues of washout media in the relief layer have an adverse effect on the printing result. The fine halftones are then higher than the uniform areas and the tonal value gain at low tonal values increases. Fine halftone vignettes can no longer be printed. An adequate drying period is accordingly indispensable.
The drying period therefore has a crucial bearing on the total time needed to make a flexographic printing plate. With the drying periods described, it is not possible to do flexographic printing jobs promptly; instead a lead time of at least one day is required before a job can be realized in a printing press. This is uneconomical in an age of ever shorter jobs, often split into numerous repeat jobs. Moreover, in the event of a flexographic printing plate being damaged, it is impossible to provide a replacement at short notice; instead the printing job has to be discontinued. The next day, all the flexographic printing plates have to be remounted before the job can be completed, which again takes time and money.
There accordingly has been no shortage of attempts to eliminate this disadvantageous property of flexographic printing forms and to shorten the drying period.
It will be appreciated that in principle the drying rate can be increased by raising the drying temperature. But attempts to accelerate the drying rate of commercially available flexographic printing plates by raising the drying temperature led to problems with plate quality. The flexographic plates dried at higher temperature gave rise to register problems in printing. Flexographic printing plates have to have an excellent level of dimensional stability because several inks are printed together. Registration accuracy, i.e., the accuracy with which the individual elements of an image are combined in printing, is about 0.1 to 0.2 mm in the case of a printed width of about one meter. Dimensional stability for a flexographic printing plate accordingly has to be better than 0.02%.
The reason for the register problems of commercially available flexographic printing plates dried at higher temperature is the thermal behavior of the polyester support sheet supporting the photopolymerizable layer of the flexographic printing plate. Commercially available polyester sheets used in the manufacture of commercially available flexographic printing plates shrink on heating to temperatures above the glass transition temperature of polyester (about 70° C.). It is accordingly necessary to set the drying temperature at below the glass transition temperature in order to foreclose any distortion or warpage due to uncontrollable shrinkage. WO 2005/121898 A1 describes a method wherein the drying of flexographic printing plates is accelerated by additional irradiation with visible light. However, this method did not yield consistent results and therefore failed to become established in the market.
Water-washable flexographic printing plates are known as an alternative to solvent-washable plates. Water-washable flexographic printing plates dry distinctly faster than flexographic printing plates washed out in organic solvents, since the boiling point of water is lower than that of organic washout agents. However, the quality and press life of water-washable flexographic printing plates is inferior to the quality of flexographic printing plates washed out in organic solvents.
Thermally developable flexographic printing plates are known as a further alternative to solvent-washable plates. In thermal development, the unpolymerized areas are heated to the point of forming a liquid melt and the melt is absorbed using an absorbent material. Thermally developable flexographic printing plates do not require a drying step. However, thermal flexographic printing plates fall far short of the quality of flexographic printing plates washed out in organic solvents.
WO 96/14603 A1 proposes photopolymerizable flexographic printing elements for thermal development which comprise a dimensionally stable, flexible, polymeric support and a photopolymerizable elastomeric layer, wherein the plate has a thermal distortion in both the longitudinal and the transversal directions which is less than 0.03% when the plate, after imagewise exposure to light, is developed at temperatures of 100 to 180° C.
The flexographic printing element is formed using support sheets composed of semicrystalline polymers. A multiplicity of different materials are recited as suitable, such as polyethylene naphthalate (PEN), polyethylene terephthalate, polyether ketones, polytetrafluoroethylene, polyamides, syndiotactic polystyrene and polyphenylene sulfide. The sheets are annealed before use.
The flexographic printing elements of WO 96/14603 A1 are formed according to the examples by annealing a PEN support sheet 0.178 mm thick at 160° C. under a defined tension. The substrates are then corona treated and coated with an aziridine primer. They are then laminated with a photopolymerizable elastomeric layer. To make flexographic printing plates, the flexographic printing elements are imagewise exposed through a negative and then thermally developed, the developer roll having a temperature of 176° C. The photopolymerizable layer is from 0.3 mm to 3 mm in thickness. WO 96/14603 A1 does not disclose processing the flexographic elements by using washout media.
“Nonshrinking” polyester sheets, having a low degree of thermal shrinkage, are known in principle and also commercially available. Sheets of this type are typically formed by extrusion from a sheet die and subsequent stretching in the machine direction and in the transversal direction. This stretching operation takes place above the glass transition temperature, as a consequence of which commercially available PET sheets usually have different shrinkage values in the machine direction (MD) and the transversal direction (TD). In order, then, to render the sheets nonshrinking they are heated to the required temperature and maintained in a tensionless state for a defined period so that shrinkage stresses present can relax. Thereafter, the PET sheet is cooled down to below the glass transition temperature before being subjected to tensile stress. PET sheets thus obtained have shrinkage values of less than 0.02%.
However, use of a nonshrinking film in the manufacture of flexographic printing elements according to existing processes does not lead to flexographic printing plates possessing excellent accuracy of register, since the sheets come under renewed thermal stress in the course of forming the flexographic printing element. The customary way to form photopolymerizable flexographic printing elements involves a process wherein the components of the photopolymerizable layer are mixed and melted in an extruder. The melt is subsequently introduced into the nip of a calender introducing the covering and supporting sheets via its heated rolls. In this process, the PET sheets come into contact with the hot photopolymerizable melt and take on the temperature of the melt.
The temperature of the photopolymerizable melt is typically in the range from 120° C. to 150° C., i.e., far above the glass transition temperature of polyethylene terephthalate. Moreover, the resulting combination of photopolymerizable layer with covering and supporting sheets has to be transported through the machine. Shearing forces will be exerted on hot PET sheets in the course of transportation. This means that, after passing through this manufacturing step, even flexographic printing plates made using nonshrinking PET sheets will again have shrinkage values above the required dimensional stability of <0.02%.
Furthermore, it is difficult to use the abovementioned method and nonshrinking PET sheets to make ripple-free flexographic printing plates. Since the tension in the machine direction is higher than perpendicularly to the machine direction, it is advantageous for the PET sheets to have a higher shrinkage in the machine direction than perpendicular thereto. Only in that way is it possible to produce flexographic printing elements that are ripple-free. Yet flexographic printing elements thus obtained will always have some residual shrinkage. It prevents attainment of the required dimensional stability on drying at elevated temperatures.